Because specific species of horses only exist at certain periods in the geological record. Every so often, a new species that looks very, very similar, but with some distinct characteristics appears where the previous one use to in the geological record. Also, off the one of the coasts of either Ireland or England, there are sheer cliffs. In these cliffs you are able to see evolutionary development of shellfish for basically a billion years as you go from the top to the bottom. 
Label: Non-Sarcastic